1. Field of the Invention
The present invention relates to a novel method for producing a stable porous polymer sheet having an excellent sponge formability and a low bounce impact elasticity.
2. Description of the Prior Art
In the production of porous polymer sheets by treating a polymer solution consisting mainly of an elastic polymer in a non-solvent for the polymer, a large number of proposals for solving problems, for example, making porous sponge structure uniform, producing such a porous sheet material without collapsing the sponge structure upon drying or shaping of the sheet or causing an extreme deformation, and making the bounce impact elasticity low without losing the inherent characteristics of the elastic polymer, have been heretofore made. These facts are important when the porous polymer sheet is used as a surface layer of a leather raw material. However, the previous processes, for example, in which in order to improve the sponge formability and to make the bounce impact elasticity low, polyurethane elastomer is used as the elastic polymer and the ratio of the hard segment and the soft segment which constitute the polyurethane elastomer is varied, the kind of polyurethane elastomer is varied, or various coagulation regulators (organic or inorganic compound or solvent composition) are together used, cannot satisfy the above described problems.
For example, the production of a soft and flexible sheet material, can be attained by selecting a polyurethane elastomer having a small amount of the hard segment but such a polyurethane elastomer is poor in coagulating ability (sponge formability) and the elasticity becomes large. The coagulating ability of the polyurethane elastomer having a small amount of hard segment can be improved to a certain degree by using various coagulation regulators but the improvement is not only insufficient, but also the elasticity cannot be lowered. Accordingly, it is very difficult to produce a soft and flexible sheet having a high drapability and high vapor permeability and moisture permeability by such a means.